Subsurface heater configuration for in situ hydrocarbon production

ABSTRACT

A configuration of elongate subsurface heaters ( 20 ) in a subsurface oil shale formation ( 10 ) for use in hydrocarbon production. The heaters ( 20 ) are grouped into a series of columns ( 40 ), and the columns extend generally perpendicular to the sedimentary layers ( 50 ) of the subsurface oil shale formation ( 10 ). The heaters may also be grouped into a series of layers, with each layer extending in one sedimentary layer of the oil shale formation.

The invention is concerned with a configuration of subsurface heaters for use in hydrocarbon production, and more particular to a configuration of subsurface heaters for heating kerogen.

Oil shale is a sedimentary rock which includes an organic mixture of complex chemical compounds, collectively referred to as “kerogen”. The oil shale consists of laminated sedimentary rock containing mainly clay minerals, quartz, calcite, dolomite, and iron compounds. Oil shale can vary in its mineral and chemical composition.

When oil shale is heated to around 350° C., pyrolysis (a destructive distillation of the kerogen) occurs to produce hydrocarbon products in the form of inter alia oil and gas. The hydrocarbon products resulting from the pyrolysis of the kerogen have characteristics that are similar to that of other petroleum products. Oil shale is considered to have potential to become an important future source of liquid fuels and natural gas, which are currently produced from other petroleum sources.

There have been a number of proposals for methods of heating the oil shale in situ to enable recovery of hydrocarbon products. For example, heat and/or solvents can be circulated through or injected into a subsurface oil shale formation. Heating methods include hot gas or liquid injection, closed loop circulation of hot gas (such as flue gas, propane, methane or superheated steam), closed loop circulation of hot liquid (such as a molten salt, water or oil), electric resistive heating, dielectric heating, microwave heating, downhole gas burners or oxidant injection to support in situ pyrolysis.

In some proposals, heating wells and production wells are provided in the subsurface oil shale formation. Hot fluid is pumped down into the heating wells, where its heat is transferred to the oil shale, and the cooled fluid is then returned to the surface. The fluid can be reheated in surface facilities before being returned to the oil shale, and if necessary, impurities in the heating fluid that have been picked up in the subsurface oil shale formation can be removed. The hydrocarbon products resulting from the pyrolysis of the kerogen can be extracted through the production wells.

In situ production of oil and gas from kerogen in oil shale has not yet been carried out commercially. Both vertical and horizontal heating and production wells are described in various publications. Various configurations and geometries of patterns of heating wells and production wells have been proposed in an attempt to optimize heating of the subsurface formation.

A problem with the heating process is that that the heating rate is very slow. Heat is transported in the subsurface formation mainly by thermal conduction, and is limited by the low thermal conductivity of the oil shale. Because of the slow heating rate, the use of a closely spaced regular pattern of heating wells has been suggested, but this leads to a high surface footprint, and increases the risk of drilling into heaters that are already in operation, Even with closely spaced heaters, it is predicted that a subsurface formation may take years to come to a suitable uniform temperature.

The invention has been made in view of the above circumstances, and it is an object of at least the preferred embodiments of the invention to increase the rate of heating of subsurface oil shale formations.

According to a first aspect of the present invention, there is provided a configuration of elongate subsurface heaters in a subsurface oil shale formation for use in hydrocarbon production, wherein the heaters are grouped into a series of columns, the columns extending generally perpendicular to the sedimentary layers of the subsurface oil shale formation.

Because the heaters are elongate and are arranged in columns, each column will generate a “wall” of heat, in that the heat flux from each heater will be augmented by the heat flux from the heaters above and below it. This provides a much larger heat flux than can be generated by a single heater, and allows the subsurface oil shale formation to be brought to a desired temperature more quickly.

The heaters can be arranged at any suitable distance from each other. In a preferred form, the distance between adjacent columns, measured along the sedimentary layers, is between 10 and 100 metres.

This provides large gaps between the columns, and production wells may be arranged between the columns of heaters without any risk of the heaters being damaged during drilling of the production wells.

In a preferred form, the heaters are arranged to extend generally along the sedimentary layers of the subsurface oil shale formation.

It is known that the thermal conductivity of a subsurface oil shale formation is larger along the sedimentary layers than across the sedimentary layers. As the heaters extend generally along the sedimentary layers of the subsurface oil shale formation, the heat flux generated will be in the preferred direction of thermal conductivity, and this will reduce the time required to bring the subsurface oil shale formation to a desired temperature.

The heaters may also be grouped into a series of layers, each layer of heaters extending in one sedimentary layer of the subsurface oil shale formation. Of course, the heaters need not extend exactly along a single sedimentary layer; for example, if the sedimentary layers are not completely planar, the heaters can approximately follow the layer. The orientation of the heaters also does not need to be perfectly aligned with the orientation of the layers, and the heaters can extend at an angle of up to 30 degrees to the sedimentary layers.

Preferably, the distance between adjacent heaters, measured across the sedimentary layers, is between 2 and 30 metres.

In a preferred form, a layer of crossing heaters is arranged in a layer above the uppermost heaters in the columns, the angle between the crossing heaters and the uppermost heaters in the columns being at least 60°, and preferably around 90°. Other layers of crossing heaters may also be provided lower in the column.

The crossing heaters will increase the temperature of the formation around them. It is known that the permeability of a subsurface oil shale formation increases with increasing temperature, and so the heated regions around the crossing heaters will be more susceptible to fluid flow. The crossing heaters thus allow products generated by pyrolysis near the columns to flow to the production wells more easily.

In a preferred form, at least one generally vertical production well is arranged between each pair of columns. There may be a single generally vertical production well, and the single well may be arranged generally equidistant from the columns, or offset towards one of the columns. With this arrangement, a single production well can be used to gather products arising from pyrolysis near two adjacent columns of heaters. It is also possible to use two production wells, one near each column, or more production wells if appropriate.

In an alternative preferred form, a production well is arranged in a layer above the crossing heaters, and extends generally parallel to the heaters in the columns.

With this arrangement, the production well will extend across the crossing heaters. Products generated by pyrolysis, flowing near the crossing heaters, can thus be gathered by a single production well, simplifying the necessary arrangement of heaters and wells.

Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying Figures, in which:

FIG. 1 is a schematic cross-sectional side view of a subsurface oil shale formation, showing an exemplary arrangement of heaters;

FIG. 2 is a schematic cross-sectional side view of a second subsurface oil shale formation, showing how the arrangement of heaters can be changed depending on the characteristics of the subsurface oil shale formation;

FIG. 3 is a schematic view of a model of an exemplary arrangement of heaters and production piping;

FIG. 4 is a schematic cross-sectional view showing heat flux in the exemplary arrangement of FIG. 3; and

FIG. 5 is a schematic view showing the order in which the various wells in the exemplary arrangement of FIG. 3 are formed.

According to a first embodiment of the invention, a series of horizontal heaters (which may be of any suitable length) are drilled into the subsurface oil shale formation in a pattern in which the heaters are grouped into columns. FIG. 1 shows a schematic cross-sectional view of the subsurface oil shale formation 10 with the heaters 20 drilled into it, and it can be seen that the heaters 20 are arranged in columns 40 which are generally parallel to each other, and generally perpendicular to the sedimentary structure 50 of the subsurface oil shale formation 10. Further, the heaters are arranged in layers 30 which extend generally parallel to the sedimentary layers 50 of the subsurface oil shale formation 10.

In a preferred form, the heaters 20 use a closed-loop circulating fluid, which can be any suitable fluid, such as molten salt, water, oil, steam, flue gas, oils, or gases. In an alternative form, the heaters 20 can use an injection fluid, and any suitable fluid can be used as the injection fluid, such as hydrocarbon liquids, gases and/or steam. In addition, electrical heating could also be used.

The heat flux from a single heater placed relatively far from other heaters is small due to the nature of thermal conduction in radial geometry. Placing a number of heaters 20 close to one another in a column 40 will create a temperature front of a more linear character, since the heat flux from each individual heater will be reinforced by the heat flux from the heaters above and below. This gives a much higher heat flux than can be generated by a single heater, and reduces the time necessary to bring the subsurface oil shale formation 10 to a suitable temperature.

Further, the thermal conductivity of the subsurface oil shale formation 10 is lower for heat flux in the vertical direction (across the sedimentary layers 50 of the subsurface oil shale formation 10) than for heat flux in the horizontal direction (along the sedimentary layers 50 of the subsurface oil shale formation 10). Some studies in which the thermal conductivities have been measured experimentally have found that the thermal conductivity along the layers is around 30% to 50% higher than the thermal conductivity across the layers. Arranging the heaters 20 as a column 40, with the heaters 20 extending generally parallel to the sedimentary layers 50, and the column 40 extending in a direction generally perpendicular to the sedimentary layers 50, will create a heat flux in the direction in which the thermal conductivity is greatest (that is, parallel to the sedimentary layers 50), and so further reduce the time necessary to bring the subsurface oil shale formation to a suitable temperature. In addition, providing a layer 30 of heaters 20 in a single sedimentary layer 50 will also increase the heat flux within that particular layer, and will also reduce the time necessary to bring the subsurface oil shale formation to a suitable temperature.

Although not shown in FIGS. 1 and 2, production wells will be placed between the heaters 20. The production wells can take any suitable form, and any combination of horizontal and/or vertical production wells, and any position of and number of production wells, may be used.

FIG. 2 shows an arrangement in which the sedimentary layers 150 of the oil shale are not horizontal throughout the entire subsurface oil shale formation 110 (which will in practice be the case). Specifically, FIG. 2 shows an arrangement with a change 160 in the direction of the layers in the subsurface oil shale formation 110, so that the sedimentary layers 150 are substantially horizontal at the left side of FIG. 2, but are angled downwards to the right at the right side of FIG. 2.

It will be seen that the layers 130 on the left side of FIG. 2 are shown as extending horizontally, along the horizontal sedimentary layers 150 of the subsurface oil shale formation 110, and the columns 140 on the left side of FIG. 2 are shown as extending vertically, perpendicular to the horizontal sedimentary layers 150 of the subsurface oil shale formation 110. However, the layers 130 on the right side of FIG. 2 extend at an angle to the horizontal, so that they still extend along the (non-horizontal) sedimentary layers 150 of the subsurface oil shale formation 110. Further, the columns 140 on the right side of FIG. 2 extend upwards at an angle to the vertical, so that they are still perpendicular to the (non-horizontal) sedimentary layers 150 of the subsurface oil shale formation 110.

It will be appreciated that when the heaters are referred to as “horizontal”, this means that the heaters extend generally along the sedimentary layers of the subsurface oil shale formation. If the sedimentary layers of the subsurface oil shale formation are not completely horizontal, but are angled with respect to the horizontal (in a direction extending into or out of the plane of the Figures), then the heaters can be formed at a corresponding angle, to ensure that they extend along the sedimentary layers.

However, the heaters need not be arranged exactly parallel with the sedimentary layers of the subsurface oil shale formation; beneficial effects can still be achieved with differences of up to 30 degrees between the orientation of the sedimentary layers and the directions of the heaters, as long as the heaters extend along the sedimentary layers.

A disadvantage of arranging a large number of heating wells in a closely spaced regular pattern is that the resulting surface footprint is very large, as each new heater requires a new well to be drilled. This problem may be overcome by arranging heaters in columns, as each column of heaters may be drilled horizontally from a single wellpad. The arrangement of columns of heaters will result in rows of closely spaced wellheads, which will be gathered in distributed well pads, and this will reduce the surface footprint.

Further, when developing a large resource such as a subsurface oil shale formation, there will be a continuous drilling programme for extending the volumes for production. It will be necessary to maintain a certain distance between heaters which are already in operation and newly-drilled heaters. Arranging the heaters in columns can help to provide the necessary safe distance for safe drilling operation, as compared to an arrangement of closely spaced, evenly distributed heaters.

The vertical distance between the layers (that is, the distance between adjacent heaters in a column measured across the sedimentary layers of the subsurface oil shale formation), and the horizontal distance between the columns (that is, the distance between adjacent columns measured along the sedimentary layers of the subsurface oil shale formation), can be varied depending on the characteristics of the subsurface oil shale formation. For example, the vertical distance may be between 2 and 30 metres, and the horizontal distance may be between 10 and 100 meters.

The applicants have carried out simulations of heat conduction as achieved by a heater configuration according to the invention, and by a different form of heat configuration. Specifically, a simulation was carried out on a columnar configuration, where the vertical distance was 4 metres and the horizontal distance was 25 meters, and on a configuration of horizontal heaters in a repeating isosceles triangular pattern with a triangle side length of 13.5 metres. These two configurations have much the same number of heaters per unit volume of the subsurface oil shale formation, and the simulation found that the subsurface oil shale formations would heat up at generally similar rates. However, the configuration according to the invention can have a lower surface footprint, and can be implemented using a simpler drilling programme, than the configuration with evenly distributed heaters.

FIGS. 3 to 5 show a further exemplary arrangement of heaters and a production well. As best seen in FIGS. 3 and 4, the arrangement consists of two vertical parallel columns 300 of horizontal heaters 310, above which is a layer of crossing heaters 320 (only one of which is visible in FIG. 4). These crossing heaters 320 are shown as extending perpendicular to the heaters 310 in the columns 300; however, the angle between the heaters does not need to be 90°. If the heaters do not cross perpendicularly, then it is preferred for the acute angle at the crossing point to be no less than 60°. The horizontal distance between two adjacent crossing heaters in the actual subsurface oil shale formation may be between 20 and 200 metres.

Above the layer of crossing heaters 320 is arranged a production well 330. In the Figures the production well 330 is shown as extending parallel to the heaters 310 in the columns 300 and perpendicular to the crossing heaters 320; however, any appropriate orientation can be used for the production well. Further, the production well 330 is placed between the columns 300 of heaters, and is preferably located generally horizontally midway between two adjacent columns.

As with the other exemplary arrangements, the “horizontal” heaters will in practice extend generally along the sedimentary layers of the subsurface shale formation, and so may not be exactly horizontal.

FIG. 4 shows the pattern of fluid flow in the arrangement of FIG. 3. As mentioned previously, a heated oil shale formation has higher permeability than a cold oil shale formation, and so fluid flow will be greater in the areas which are warmer. As FIG. 4 shows, fluid flows upwards through the sedimentary layers near the columns 300 of heaters 310, until it reaches the crossing heaters 320, The fluid will then tend to flow in the direction of the crossing heaters 320, as these have heated the surrounding reservoir and increased its permeability. The fluid will then enter the production well 330, and can be extracted from the reservoir.

FIG. 5 shows the order in which the various wells and heaters are preferably formed in the third exemplary embodiment.

Firstly, the production well 330 is formed. Secondly, the layer of crossing heaters 320 underneath the production well 330 is formed, and operation of this first layer of crossing heaters 320 is commenced, to start to heat the subsurface oil shale formation.

Then, the vertical parallel columns 300 of horizontal heaters 310 are formed, and operation of these heaters 310 is commenced, to heat other regions of the subsurface oil shale formation.

The regions near the crossing heaters 320 will undergo pyrolysis to produce hydrocarbon products before the rest of the subsurface oil shale formation, as they are the first regions to be heated. Extraction of hydrocarbon products resulting from pyrolysis in the regions near the crossing heaters 320 can begin while the vertical parallel columns 300 of horizontal heaters 310 are still heating the subsurface oil shale formation (and, in some cases, even before the columns 300 have been completed), and so extraction of the hydrocarbon products can be started earlier. The heat supply to the crossing heaters can be stopped once extraction of the hydrocarbon products has started.

FIG. 5 shows two columns 300 of heaters 310 and the production well 330 being arranged in a “block”; several blocks can be arranged side by side in a repeating pattern in the subsurface oil shale formation.

Further, FIGS. 3 to 5 show an arrangement with a single layer of crossing heaters, arranged between the top layer of the vertical parallel columns of horizontal heaters and the production well. However, it would of course be possible for the layer of crossing heaters to be placed further down in the columns. It would also be possible to provide more than one layer of crossing heaters, so that the heaters would have a lattice arrangement. The exact details of the arrangement of heaters will of course depend on the particular details of the subsurface oil shale formation. 

1. A configuration of elongate subsurface heaters in a subsurface oil shale formation for use in hydrocarbon production, wherein the heaters are grouped into a series of columns, the columns extending generally perpendicular to the sedimentary layers of the subsurface oil shale formation.
 2. A configuration of subsurface heaters as claimed in claim 1, wherein the distance between adjacent columns, measured along the sedimentary layers, is between 10 and 100 metres.
 3. A configuration of subsurface heaters as claimed in claim 1, wherein the heaters are arranged to extend generally along the sedimentary layers of the subsurface oil shale formation.
 4. A configuration of subsurface heaters as claimed in claim 3, wherein the heaters are also grouped into a series of layers, each layer of heaters extending in one sedimentary layer of the subsurface oil shale formation.
 5. A configuration of subsurface heaters as claimed in claim 3, wherein the distance between adjacent heaters, measured across the sedimentary layers, is between 2 and 30 metres.
 6. A configuration of subsurface heaters as claimed in claim 1, wherein production wells are arranged between the columns of heaters.
 7. A configuration as claimed in claim 1, wherein a layer of crossing heaters is arranged in a layer above the uppermost heaters in the columns, the angle between the crossing heaters and the uppermost heaters in the columns being at least 60°, and preferably around 90°.
 8. A configuration as claimed in claim 7, wherein at least one generally vertical production well is arranged between the columns.
 9. A configuration as claimed in claim 7, wherein at least one generally horizontal production well is arranged in a layer above the crossing heaters. 